Want to switch rear ends?

[woodman5455]

I have a 1999 Freightliner FL70 with a 4:10 rear. I am going Friday to pick it up after having a recon 240hp cummins engine has been installed. When I get out to the west coast where the speed limit is 75 I know in the past that at 2,400 rpm I could only do about 73 mph. I was thinking about changing the rear this summer during my slow season ( I do trade shows from Oct. to May ).

I just feel that running the engine at the max rpm can not be good for it. Does any one have a input on what I should do? I have a 6 speed Transmision.
Thanks
Dale

 

[Glen Rice]

Sounds resonable to change your final drive ratio. There are a couple of ways to do it. If you are running low profile tires you could go to tall rubber, 11R 22.5 for example. Maybe a transmission swap to a overdrive top gear would do the trick, if your trans was in need of overhaul. And then as you stated the gear ratio swap, don't get to crazy as you will loose some of your gradability, this is your trucks ability to climb a hill. You don't want to have to downshift at the sight of a hill ahead sign!! Don't be afraid to ask a dealer to help you with your options. You want to do this correctly the first time. Good luck.

 

[RichM]

It would probably be cheaper just to run at a slower speed,mileage would also be better at 65 vs 75..

 

[UpNorth_10]

I changed the rear end in my FL70 a couple of months ago. I went the other way though. I went from a 3.90 to a 4.33. I wanted to be able to handle the grades a little better. The tranny only has direct final drive, no overdrive so I only get a top speed of about 60mph.

Are u going to do the swap yourself or hire someone? I bought the ring and pinion plus all new bearings and did it myself.

You'll have to reset the dip switches on the speedo head to get correct a correct mph reading.

If your truck has plenty of torque for the mountains currently, I would put a new tranny in with a overdrive gear or even a double overdrive instead of a new rear. The cost would be similar. I know you can buy a 10 speed for these smaller motors.

UpNorth

 

[simon says]

Hi- I think you're between a rock and a hard place. Sounds like you have a direct drive 6 speed (if your rpms are 2400+ @73) and you already have tall rubber. By slowing down to 65 mph, you will lower rpms by about 200, and that will help a lot. But you are still "gear-bound" by that transmission. 1)IF- you had the money to spend and you have committed to that truck, I would replace that transmission with an overdrive 10 speed and leave the rear end. 2)If you swap out the rear end to 3.9 or even 3.73, you should have enough startability and you drop rpms by 100-200. My first truck was a 9 spd direct and a 275 hsp Cat 3126, and it would only do 70 mph @ 2400+. I got rid of it within 1.5 years. Try Cummins Powerspec for an ISL motor, and you will find the best set-up (their ISC set-up is dated and wrong).